If you are using the Sametime 8.5.1 web client for the iPhone and only the Sametime logo is visible and there is no prompt for user and password to login, then you might have disabled guest logins for web clients.
This issue is fixed in 188.8.131.52. If you need to get it working in 8.5.1 you need to enable guest login for Sametime web clients. To do this, edit the file "stproxyconfig.xml" which can be found in "<WAS server>\profiles\<profilename>\config\cells\<cellname>\nodes\<nodename>\servers\STProxyServer>".
You need to add the parameter
to the <server> section.
If you have a clustered installation make sure to edit the file on the deployment manager and to synchronize your nodes afterwards. Then restart the proxy server.
Now the login prompt should appear correctly.